The present invention relates to examining the uniformity of properties of a superconducting thin film, and in particular, to non-destructively measuring the local critical current density of the film as a function of temperature and location.
There is no simple, nondestructive, method currently available that can measure the uniformity of the electrical properties of a superconducting thin film over a large area. In particular, for microwave applications, one needs to have an extremely low surface impedance over the entire useful area of the film. Direct measurement of the surface impedance requires a fairly large area, typically several centimeters in diameter, and therefore is of limited use for determining uniformity. On the other hand, one typically finds that a low surface impedance, at microwave frequencies, correlates with a high critical current density. The critical current density can be measured at a low frequency. Therefore one can use a device for measuring the uniformity of the critical current density as a way of measuring the uniformity of the surface impedance.
Other techniques give a single value to such quantities as the surface impedance (by measuring the Q of a cavity which utilizes the superconducting film as one end wall), or the surface impedance of a stripline. The first technique is an average over a large area, while the second requires the film to be patterned and is thus destructive.
One technique for determining the critical current density of a small section of a superconducting film is to put a small (1 to 4 mm diameter) coil in close proximity to the film. Passing an ac. current through the coil induces an ac. current in the film. By measuring the third harmonic response as a function of the coil's current one can determine the critical current of the film (see "A contactless method for measurement of the critical current density and critical temperature of superconducting film" J. H. Classen, M. E. Reeves, and R. J. Soulen, Jr., Rev. Sci Inst. vol. 62, pp. 996-1004, 1991). This works because the current induced in the film is proportional to the current in the coil and because a third harmonic signal occurs when the film leaves the superconducting state due to the induced current exceeding its critical current carrying ability. Thus, one can, after calibration, make a direct measurement of the critical current density.
The need for determining film quality and uniformity of superconducting films is important in developing phased array superconducting microwave antenna.
It is believed that there is no process for measuring the uniformity of the electrical properties of a superconducting thin film in such a way as to avoid destroying the film. Thus, there exists a need for the development of the present invention which is able to examine the uniformity of the electrical properties of a superconducting thin film without harming said film.